People often get confuse with this term .To start with, it will be a good idea to know the difference between Fusion Applications and Middleware. Oracle Fusion is composed of two parts: Fusion middle ware and Fusion applications.

Oracle Fusion Middle ware aka FMW : Comprises of the Oracle Application Server and other technology stack components that Oracle has acquired in past few years.

Fusion Middleware is a family of middleware products covering areas like BI, Identity Management, Content Management and SOA.

Each of these areas has it's own stack of products. Fusion Middleware was previously called Oracle Application Server.

Oracle Fusion Applications ....you can called as ERP module for Fusion : Oracle Fusion Applications is Oracle's next generation suite of applications that eventually replace E-Business Suite. It will assimilate best of breed features from:

E-Business Suite

JD Edwards

PeopleSoft

Siebel.

Fusion Applications is build on top of the Oracle Fusion Middleware technology stack using Oracle's Fusion Architecture as blueprint.

Therefore Fusion you can be refer to any of three things Middleware, Application, Architecture.With this Oracle is not only moving to the next generation technology stack like SOA, BPEL, BAM, JSF, Ajax, ESB, etc, but will also bring together the best in class underpinnings, of each of the incumbent suites, like Business Structure, Security Model, Workflow Process, and Event Models which you will find in later part of discussion.Before let have a quick look on Oracle's Fusion Architecture .

ORACLE FUSION ARCHITECTURE

Oracle Fusion Architecture (OFA) is a standards-based technology reference architecture or blueprint for building applications. Note that Oracle Fusion Architecture is not a product, and can be used without licensing it from Oracle. Many organizations use it to write applications that can run on Oracle Fusion Middleware or that compliment Oracle Fusion Applications.[Adopted http://www.oracle.com]

Oracle Fusion Architecture is based on the following core principles:

Model Driven: For applications, business processes and business information

Hoping Oracle announced the general availability of Oracle Fusion Applications in the sometime in first quarter of this year

Fusion V1 & Fusion V2 functionality

Fusion V2 will be more likly to be more focused for Manufacturing, Supply Chain gaps, more localizations,and public sector functionality

MIGRATION TO ORACLE FUSION APPLICATIONS

The Oracle Fusion Applications platform was built with flexibility in mind,so companies can selectively add modules if and when it makes business
sense.

With the initial release of Oracle Fusion Applications likely focused on point solutions, most users will be concerned with how to integrate Oracle Fusion Applications into their core applications. This will be a substantially easier effort than migrating the entire core functionality

There are options/proposed path which can observed in various Oracle papers.

Here’s a plan for IT side which may be consider to be crafted roadmap for the future of enterprise applications

Continue on Your Current Path .That mean you need to evaluate the latest Applications Unlimited releases Oracle Applications releases such as Oracle E-Business Suite 12.1.3, PeopleSoft Enterprise 9.1, Siebel Customer Relationship Management 8.2, and JD Edwards EnterpriseOne 9.1 for the additional value they bring to the businesse .In addition to gaining value today, you will be in a position to upgrade to Oracle Fusion Applications in the future, if and when you choose.

EBS -> Fusion Apps

PeopleSoft Financials to Fusion Apps

PeopleSoft HRMS to Fusion Apps

Siebel CRM to Fusion Apps

Move to Fusion Step by Step . You need to adopt standards-based technology first. Consider Oracle Fusion Middleware as inhouse new initative and developing skill sets in technologies such as Oracle Application Development Framework, Oracle content management products, Oracle SOA Suite, and Oracle WebCenter. These tools will help you integrate, extend, and manage your applications environment today and prepare you to add Oracle Fusion Applications if and when you choose by upgrading one environment and then reimplement rest what ever/when ever.

Upgrade to FusionApplications . Decide(if and when appropriate.)
If you use any functionality that is not part of V1 but still see business value in going to Fusion Apps, this may result in multiple Fusion Apps instances.